ACP--->General--->Categories and Forums---> Then click the modify button which should look like this (this is only for an existing forum). If its for a new forum then create forum as usual. then when you scroll down you'll see an option to add an Image URL into a box, you add this URL of the image.

To get the image of a URL there is several ways.

Right clicking on an image while using google chrome allows you to copy the image urlUploading an image to a image hosting website such as Photobucket or servming and then once you've uploaded the image you'll see a bit to copy an image url.

Okay, so the only thing holding this back would be the fact you have an icon towards the left.

To remove this you need to go into your admin control panel.

Go to Display--Pics Management--Advanced Mode.

Now scroll down until you see the image that is blocking the picture from sitting right which is this (I took this from the picture you provided above) then remove the link and click save. I suggest copying the link to a document encase you need it at some point in the future.